This study assessed the effects of 24-epibrassinolide (EBL, 10–7M) and silicon (2 mM) on the alleviation of cadmium (Cd, 150 mg L–1) toxicity in Pisum sativum L. seedlings via the modulation of growth, antioxidant defense, glyoxalase system, and nutrient uptake. Shoot and root lengths declined by 46.43% and 52.78%, respectively, following Cd stress. Shoot and root dry weights also declined with Cd toxicity. Biochemical and physiological aspects exhibit significant decline including total chlorophyll (33.09%), carotenoid (51.51%), photosynthetic efficiency (32.60%), photochemical quenching (19.04%), leaf relative water content (40.18%), and gas exchange parameters (80.65%). However, EBL or Si supplementation alone or in combination modulates the previously mentioned parameters. Cadmium stress increased proline and glycine betaine (GB) contents by 4.37 and 2.41-fold, respectively. Exposure of plants to Cd stress increased the accumulation of H2O2, malondialdehyde content, electrolyte leakage, and methylglyoxal, which declined significantly with EBL and Si supplementation, both individually and in combination. Similarly, Cd stress adversely affected enzymatic and non-enzymatic antioxidants, but EBL and/or Si supplementation maintained antioxidant levels. Glyoxalase I (GlyI) accumulated after Cd stress and increased further with the application of EBL and Si. However, GlyII content declined after Cd stress but increased with supplementation of EBL and Si. Cadmium accumulation occurred in the following order: roots > shoots>leaves. Supplementation with EBL and Si, individually and in combination reduced Cd accumulation and enhanced the uptake of macronutrients and micronutrients in shoots and roots, which declined with Cd toxicity. The application of 24-EBL and Si, individually and in combination, alleviated the adverse effects of Cd by improving growth, biochemical parameters, nutrient uptake, osmolyte accumulation, and the anti-oxidative defense and glyoxalase systems in Pisum sativum seedlings.

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Interactive effect of 24-epibrassinolide and silicon alleviates cadmium stress via the modulation of antioxidant defense and glyoxalase systems and macronutrient content in Pisum sativum L. seedlings.

Plant-lead interactions: Transport, toxicity, tolerance, and detoxification mechanisms.

Chromium (Cr) is one of the top seven toxic heavy metals, being ranked 21st among the abundantly found metals in the earth's crust. A huge amount of Cr releases from various industries and Cr mines, which is accumulating in the agricultural land, is significantly reducing the crop development, growth, and yield. Chromium mediates phytotoxicity either by direct interaction with different plant parts and metabolic pathways or it generates internal stress by inducing the accumulation of reactive oxygen species (ROS). Thus, the role of Cr-induced ROS in the phytotoxicity is very important. In the current study, we reviewed the most recent publications regarding Cr-induced ROS, Cr-induced alteration in the enzymatic antioxidant system, Cr-induced lipid peroxidation and cell membrane damage, Cr-induced DNA damage and genotoxicity, Cr-induced ultrastructural changes in cell and subcellular level, and Cr-induced alterations in photosynthesis and photosynthetic apparatus. Taken together, we conclude that Cr-induced ROS and the suppression of the enzymatic antioxidant system actually mediate Cr-induced cytotoxic, genotoxic, ultrastructural, and photosynthetic changes in plants.

Chromium-Induced Reactive Oxygen Species Accumulation by Altering the Enzymatic Antioxidant System and Associated Cytotoxic, Genotoxic, Ultrastructural, and Photosynthetic Changes in Plants.

Toxic effects of heavy metals Pb and Cd on mulberry (Morus alba L.) seedling leaves: Photosynthetic function and reactive oxygen species (ROS) metabolism responses.

Toxic effects of heavy metal Cd and Zn on chlorophyll, carotenoid metabolism and photosynthetic function in tobacco leaves revealed by physiological and proteomics analysis.

In this study, we investigated the possible role of hemin in alleviating zinc (Zn), lead (Pb) and chromium (Cr) toxicity in rice seedlings grown hydroponically by analyzing the morphological and physiological parameters. Our results showed that exposure of rice seedlings to excess Zn, Pb or Cr could cause severe leaf chlorosis, inhibit photosynthetic activity and consequently suppress plant growth. The concentration of O2
 •− and H2O2 significantly increased and the activities of antioxidative enzymes decreased in roots of rice seedlings under metal exposure. The combined treatments (hemin + ZnSO4, hemin + Pb(NO3)2 and hemin + K2Cr2O7), on the other hand, significantly enhanced the photosynthesis- and plant growth-related parameters compared with their corresponding heavy-metal-stress alone. Combined treatments dramatically stimulated the activities of superoxide dismutase (SOD), ascorbic peroxidase (APX) and glutathione reductase (GR) as well as the concentrations of ascorbic acid (AsA) and glutathione (GSH) as compared with the metal- stress alone. The concentrations of reactive oxygen species (ROS, e.g. O2
 •− and H2O2) were significantly reduced in the metal plus hemin treatments. Hemin addition also reduced metal accumulation in rice seedlings especially in root tissues. These findings suggest that hemin-elevated levels of antioxidants, activities of antioxidative enzymes and hemin-reduced accumulation of heavy-metal could confer resistance against Zn, Pb, and Cr stress in rice seedlings, resulting in improved pigments accumulation, photosynthetic attributes and plant growth.

Hemin-mediated alleviation of zinc, lead and chromium toxicity is associated with elevated photosynthesis, antioxidative capacity; suppressed metal uptake and oxidative stress in rice seedlings

Several pharmacological experiments were performed to investigate the possible roles of heme oxygenase 1 (HO1) and its catalytic products on hemin-induced attenuation of excess zinc toxicity. The results showed that hemin markedly reduced concentrations of zinc in rice seedlings (including roots and shoots); this effect may be achieved by down-regulating the relative expression of OsZIP1, OsZIP3, OsZIP7, and OsZIP8. However, zinc protoporphyrin IX (Znpp, an HO1 inhibitor) application could reverse hemin-induced decreases in zinc accumulation and upregulate the relative expression of the above four genes. These results showed that HO1 is indeed involved in the mitigation of zinc stress. The following experiments were conducted to illustrate the contributive effect of different hemin by-products in the hemin-mediated attenuating effect. The results showed that bilirubin (BR) does not have any alleviative effect, whereas carbon monoxide (CO) has a similar effect as hemin; the alleviative effect of ferrous ion (Fe2+) is even better than that of hemin. Altogether, these findings suggested that hemin, through the HO1/Fe2+, CO system, may reduce zinc accumulation in rice seedlings and ultimately strengthen Zn tolerance of rice seedlings.

Hemin Through the Heme Oxygenase 1/Ferrous Iron, Carbon Monoxide System Involved in Zinc Tolerance in Oryza Sativa L.

Hydrogen gas (H2) was recently proposed as a therapeutic antioxidant and signaling molecule in clinical trials. However, the underlying physiological roles of H2 in plants remain unclear. In the present study, hydrogen-rich water (HRW) was used to characterize the physiological roles of H2 in enhancing the tolerance of Brassica campestris against cadmium (Cd). The results showed that both 50 μM CdCl2 and 50%-saturated HRW induced an increase of endogenous H2 in Brassica campestris seedlings, and HRW alleviated Cd toxicity related to growth inhibition and oxidative damage. Seedlings supplied with HRW exhibited increased root length and reduced lipid peroxidation, similar to plants receiving GSH post-treatment. Additionally, seedlings post-treated with HRW accumulated higher levels of reduced glutathione (GSH) and ascorbic acid (AsA) and showed increased GST and GPX activities in roots. Molecular evidence illustrated that the expression of genes such as GS, GR1 and GR2, which were down-regulated following the addition of Cd, GSH or BSO, could be reversed to varying degrees by the addition of HRW. Based on these results, it could be proposed that H2 might be an important regulator for enhancing the tolerance of Brassica campestris seedlings against Cd, mainly by governing reduced glutathione homeostasis.

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Qin Chen

Papers

Hemin-mediated alleviation of zinc, lead and chromium toxicity is associated with elevated photosynthesis, antioxidative capacity; suppressed metal uptake and oxidative stress in rice seedlings

Hemin Through the Heme Oxygenase 1/Ferrous Iron, Carbon Monoxide System Involved in Zinc Tolerance in Oryza Sativa L.

Cadmium-Induced Hydrogen Accumulation Is Involved in Cadmium Tolerance in Brassica campestris by Reestablishment of Reduced Glutathione Homeostasis.